Chinese health officials say at least 60 percent of SARS cases in Beijing cannot be traced to the source of infection, raising new concerns on how to contain the outbreak. While Taiwan Friday reported its single highest increase in patients with Severe Acute Respiratory Syndrome. Liang Wannian, deputy chief of Beijing's Health Administration, said Friday his agency does not know how the majority of new Severe Acute Respiratory Syndrome infections in China's capital, Beijing, were contracted. "According to our statistics 40 percent of the newly received SARS patients have been in quarantine or under medical observation," he said. "But for the rest of the patients we do not know how they got infected. We do not know where they got infected. They just popped up in the hospital as confirmed SARS patients."

International experts say "contact tracing" of SARS patients, which determines exactly whom infected people have had contact with, is key to stemming the outbreak.

But Beijing officials say they are still optimistic new infections in the city will continue to decline. Beijing recorded 48 new cases Friday. In April the daily number of new infections ranged between 60 and 80.

China's capital has the world's largest outbreak of SARS. Nationwide China now has more than 4,800 infections and 230 people have died.

In Taiwan Friday - a day after the World Health Organization extended a travel advisory to Taiwan's capital - health authorities in Taipei reported 18 new cases for a total of 149. That is the city's biggest single day rise in SARS infections.

Hong Kong, which has more than 1,600 cases, on Friday reported just six new cases - its lowest number of new infections since the disease started spreading rapidly in mid-March.

Hong Kong's leader Tung Chee-hwa announced the news to reporters.

He says the downward trend in SARS cases was encouraging and that while 210 have died, more than 1,000 have recovered.

Worldwide the disease, which causes a serious pneumonia, has hit more than 7,100 people.

The WHO Thursday doubled its estimate of the global death rate from SARS to between 14 and 15 percent.

WHO said it made the revisions based on the latest data from Canada, China, Hong Kong, Singapore and Vietnam. This comes after an international group of scientists published a study showing the disease is killing 20 percent of all patients in Hong Kong.